Recently, paste compositions comprising resin binders in which inorganic fine particles such as conductive powders and ceramic powders are dispersed have been used to obtain sintered bodies in various shapes. Particularly, phosphor paste compositions comprising resin binders in which phosphors as inorganic fine particles are dispersed have been used for PDP (Plasma Display Panel), FED (Field Emission Display), SED (Surface Conduction Electron Emission Display), and the like and demands for them have been increasing in these years.
PDP is a display for carrying out display action by exciting phosphors and emitting phosphors by ultraviolet rays generated by gas electric discharge. In PDP, an ultraviolet-excitation type phosphor layer is formed in an electric discharge space and the phosphor layer is excited by the electric discharge to display with a color different from the light color emitted by the electric discharge gas. Generally, PDP for color display has phosphor layers of three colors; red (R), green (G), and blue (B).
Conventionally, each phosphor layer of R, G, and B is formed by processing a phosphor paste composition, mainly comprising phosphor particles in powder state and ethyl cellulose, into a prescribed shape every color by applying method using such as a screen printing method or a doctor blade method; or by a casting method in order to process the paste composition into a sheet, and successively firing the paste composition. Among them, the screen printing method is a method suitable for mass production. However, ethyl cellulose is inferior in thermal decomposability and requires a high temperature for completely remove an organic component and therefore, binder resin compositions excellent in low temperature decomposability have been developed.
For example, Patent Document No. 1 discloses a method of forming a phosphor layer for each of R, G, and B by applying a phosphor paste composition comprising a specified resin binder and a phosphor to a substrate by the screen printing method, pre-baking the paste composition after the application, thereby removing the organic solvent, carrying out exposure and development, and finally firing the composition.
Further, Patent Document No. 2 discloses a method of forming a phosphor layer by transferring a phosphor paste composition in form of a film containing phosphor particles, a (meth)acrylic type resin binder, and glycerin-1,2-diacetyl-3-monolaurate to a substrate and firing the transferred resin composition layer.
To carry out the screen printing well, it is required for the paste composition to have a so-called thixotropic property (hereinafter, also referred to as thixotropy), that is a sufficiently low viscosity and be easy for application at the time of application and contrarily that is a sufficiently high viscosity to avoid spontaneous flow out at the time the paste composition is kept still and dried after application. Herein, the thixotropy means a property, that is, in the case the viscosity is evaluated by a rotary viscometer, the viscosity becomes low when the rotation speed is high (displacement at a high strain rate) and the viscosity becomes high when the rotation speed is low (displacement at a low strain rate).
In consideration of the process of printing a paste composition on a substrate by screen printing and successively thermally decomposing a binder composition in the paste composition by heating and firing to form a layer of an inorganic component, which has been previously dispersed in the paste composition, various investigations have been made to provide thixotropy with an acrylic resin binder. For example, Patent Document No. 3 discloses a paste composition containing silica fine particles in order to improve the thixotropy. However, since the silica fine particles are produced by a sol-gel method or the like from a row material such as tetraethoxysilane or the like, the silica fine particles have a reactive functional group on the surfaces and tend to coagulate one another. Accordingly, the silica fine particles are inferior in dispersibility and also in the storage stability because of proceeding of the coagulation with the lapse of time and it results in a problem that the screen printability is deteriorated with the lapse of time.
Further, since the silica fine particles are not decomposed even by firing, they remain in a final product.
Patent Document No. 1: Japanese Kokai Publication Hei-9-208640
Patent Document No. 2: Japanese Kokai Publication 2003-96305
Patent Document No. 3: Japanese Kokai Publication 2000-144124